1. Field of the Invention
The present invention relates to an optical pickup lens and an optical pickup apparatus used for a recording and reproduction apparatus of an optical disc. More particularly, the present invention relates to an optical pickup lens and an optical pick up apparatus which is used for a compatible optical disc apparatus compatible with different types of optical discs such as Compact Disc (CD) including CD-R, Digital Versatile Disc (DVD), Blu-ray and High-Definition DVD (HD-DVD).
2. Description of Related Art
Compatible optical disc apparatus capable of recording or reproducing different types of optical discs such as CD and DVD in one system have been proposed.
In order to record or reproduce information signals stored on media such as CD and DVD (which are hereinafter collectively called the “optical disc”), the compatible optical disc apparatus needs to focus a laser beam from a light source on an information recording surface of each optical disc through a transparent substrate. However, the wavelength differs between a laser beam used for recording or reproducing CD and a laser beam used for recording or reproducing DVD. Further, the thickness of a transparent substrate also differs between CD and DVD; specifically, CD having a transparent substrate of 1.2 mm while DVD having a transparent substrate of 0.6 mm. Thus, if an optical pickup lens which has been used for focusing a laser beam on an information recording surface of each optical disc through a transparent substrate is used in common for CD and DVD in the compatible optical disc apparatus, it is unable to focus the laser beams used for CD and DVD close to the diffraction limit respectively on the information recording surface of the corresponding optical discs due to aberrations caused by these differences.
Recently, an optical disc apparatus compatible with optical discs capable of ultra high-density recording, such as Blu-ray and HD-DVD, has been proposed. Future compatible optical disc apparatus are expected to be capable of recording or reproducing not only CD and DVD but also optical discs allowing ultra high density recording. Thus, though a conventional compatible optical disc apparatus allows for two different light source wavelengths and two different thicknesses of transparent substrates, a future compatible optical disc apparatus needs to allow for at most three different light source wavelengths and at most three different thicknesses of transparent substrates.
An approach to meet the need for compatibility with at most three different light source wavelengths and at most three different thicknesses of transparent substrates is to provide an optical disc apparatus with a plurality of optical pickup lenses that prevent the occurrence of aberration for different types of optical discs and replace the optical pickup lenses in accordance with the type of the optical disc in use. Another approach is to use a plurality of optical pickup apparatus for different types of optical discs and replace the optical pickup apparatus in accordance with the type of the optical disc in use. However, in terms of cost and size reduction, it is preferred to use the same lens for any type of optical discs.
An example of an optical pickup lens compatible with a plurality of types of optical discs is disclosed in Japanese Unexamined Patent Application Publication No. 11-287948. This optical pickup lens can suppress the aberration which occurs when reproducing CD without substantially sacrificing the reproducing characteristics when reproducing DVD by forming a plurality of steps of loop zonal recess and raised portions, thus being compatible with two types of optical discs (DVD and CD) with one condenser lens. In this optical pickup lens, the depth or height (which is referred to herein as the “step height”) h of the loop zonal recess or raised portion is set so that a difference in optical path length between the part without the loop zonal recess or raised portion and the part with the loop zonal recess or raised portion is substantially an integral multiple of the wavelength for DVD. Specifically, a unit step height h of the loop zonal recess or raised portion is expressed as h˜m*λ1/(n1−1) where m represents a natural number, λ1 is a light source wavelength for DVD, and n1 represents a refractive index of an optical pickup lens.
Since the above optical pickup lens can record or reproduce a plurality of types of optical discs, it eliminates the need for a unit to replace optical pickup lenses or optical pickup apparatus for each optical disc, thus enabling cost reduction and structure simplification.
However, the optical pickup lens disclosed in the above-mentioned Japanese Unexamined Patent Application Publication No. 11-287948 has two drawbacks. First, this optical pickup lens has a plurality of steps of loop zonal recess and raised portions formed on its surface in order to reduce wavefront aberration for DVD and CD and thereby achieves sufficient reduction of wavefront aberration for DVD; however, it cannot necessarily achieve practically sufficient reduction of wavefront aberration for CD.
Second, this optical pickup lens is designed only in light of DVD and CD, and not in light of optical discs compatible with ultra high-density recording, such as Blu-ray and HD-DVD, which have been developed recently. It is a significant issue for future compatible optical disc apparatus to be capable of not only recording or reproduction of DVD and CD but also recording or reproduction of optical discs compatible with ultra high-density recording, such as Blu-ray and HD-DVD.
The present invention has been accomplished to solve the above drawbacks and an object of the present invention is thus to provide an optical pickup lens and an optical pickup apparatus which can exert the effect of practical and sufficient reduction in wavefront aberration for at least three types of optical discs. More specifically, the object of the present invention is to provide an optical pickup lens and an optical pickup apparatus which enable the practical and sufficient reduction in wavefront aberration for both DVD and CD and which are compatible with optical discs for ultra high-density recording such as Blu-ray and HD-DVD.